It's thanks to a rare event called iterative evolution, in which the process of evolution from a same ancestor is repeated along different points in history.

In this case, the white-throated rail — a bird indigenous to Madagascar — had evolved to become flightless in different occasions separated by a few thousand years.

Around 100,000 years ago, sea levels fell due to an ice age, allowing the island of Aldabra to exist once again. The rails, which migrated to other islands including Mauritius and Reunion, also landed on Aldabra.

Aldabra, then free of predators, allowed the rail to evolve and become flightless due to the lack of threats on the island. You can still find the last surviving colony of rails on the island today.

Researchers in the study, who published their findings in the Zoological Journal of the Linnean Society, studied the rail's fossils from before and after the flooding event. They found that the wing bones were in an advanced state of flightlessness, while bones from the ankle showed signs that it was evolving toward flightlessness.

"These unique fossils provide irrefutable evidence that a member of the rail family colonised the atoll, most likely from Madagascar, and became flightless independently on each occasion," lead researcher Julian Hume said in a statement.

"Fossil evidence presented here is unique for rails, and epitomises the ability of these birds to successfully colonise isolated islands and evolve flightlessness on multiple occasions.”

David Martill, a co-author on the study, said they know of no other example of rails or birds generally, who've exhibited the phenomenon of iterative evolution. Sabretooths, with their impressive teeth, have benefited from this process over the course of history.

"Only on Aldabra, which has the oldest palaeontological record of any oceanic island within the Indian Ocean region, is fossil evidence available that demonstrates the effects of changing sea levels on extinction and recolonisation events," he added.

"Conditions were such on Aldabra, the most important being the absence of terrestrial predators and competing mammals, that a rail was able to evolve flightlessness independently on each occasion."